I've only had one breakup since becoming poly, and I got nothing but support from my other partner. It just happened that my ex alienated her around the same time that we broke up.
Now I am in a triad. I didn't mean to be, but I fell for my metamour! Although I have a hard time imagining my partners breaking up, it's very difficult for me when they argue. I try hard not to take sides but I want to be supportive and listen if they need to vent.